Studies of the natural history of La Crosse (LAC) California group arbovirus will be continued in suburban and rural hardwood deciduous forested areas and parks where children are being infected year after year in Wisconsin and other mid-western and eastern states. Distribution of LAC virus activity will be determined by detection of antibodies in mammalian hosts, including chipmunks, squirrels and sentinel rabbits, and by prospective studies using antibody acquisition rates. Improved methods for rapid detection of LAC virus in the vector Aedes triseriatus will be developed. Fluorescent antibody technique will be used to visualize LAC virus in smears and frozen sections of Aedes triseriatus eggs, larvae, and adults. These methods will be used to delineate virus maintainence mechanisms involving overwintering and continuing summer season emergence of LAC virus from natural oviposition sites in basal tree- holes, old tires, and other water holding containers in these forested parks and suburbs. Epidemiologic studies of cases of California encephalitis will be conducted for evidence of the circumstances of infection.